As is known in the art, automotive catalysts have been used for many years to effectively remove pollutants generated by internal combustion engines. One of the key functions of a catalyst is to become active at low temperatures (light-off) in order to quickly control cold engine start-up emissions. Cold-start hydrocarbon (HC) emissions, generated in the first 20 seconds following cold engine start-up, constitute the majority of the HC emissions in vehicle exhaust. Many different technologies have been considered to improve the cold-start performance of catalysts including: Close-Coupled locations (catalyst close to engine), PETA (port exhaust thermactor air—running a vehicle rich and adding air), HC traps, high cell density/low thermal mass catalysts, high PGM loading, zone-coating washcoats, electrically heated catalysts, etc). All the described technologies are associated with increased complexity, cost and/or extra wear on the catalyst. Various vehicle cold-start calibrations and strategies (spark control, air/fuel (A/F) control, charge motion control, VCT, etc.) have also been employed to improve catalyst light-off. Calibration methods for improving catalyst performance can be desirable since they do not introduce additional hardware or much cost to the vehicle. Yet calibration for cold-start emissions performance is limited in that it has to be seamless to the customer, or vehicle operator, and they tend to reduce Nitrogen Oxide (NOx) or HC and Carbon Monoxide (CO) emissions, but not both simultaneously.
The inventors have recognized that one way to improve the cold start performance of a catalyst is by increasing the available energy, or enthalpy entering the catalyst, to improve catalyst light-off. The inventors have discovered that imposing a pre-determined level of A/F modulation during the cold start portion of catalyst evaluation results in increased levels of H2 in the combustion exhaust and significantly lower light-off temperatures for the catalyst, independent of overall A/F. More particularly, the inventors have discovered that the air/fuel ratio modulation produces a relatively greater concentration of H2 in the engine exhaust compared to no modulation independent of the operating condition of the engine, (i.e., during net rich, lean, or stoichometric engine operation). Because H2 is a very reactive component (very low light-off temperature), it accelerates the reaction of other monitored components (HC, CO, NOx) by providing additional heat. Excess H2 can also react with NOx and SOx (H2+NOx→H2O+N2) for enhanced LNT or TWC regeneration function.
In accordance with the present invention, a method is provided for controlling an internal combustion engine wherein, during an initial, relatively low temperature operating phase of the engine, the engine operates with a modulated air/fuel ratio established independently of the operating condition of the engine and wherein, during a subsequent higher temperature operating phase of the engine, the air/fuel ratio is modulated in as a function of the operating conditions of the engine.
In one embodiment, the initial, relatively low temperature is before light-off.
In one embodiment, the engine includes a system for determining the air/fuel ratio operating condition of the engine and wherein such modulation is independent of determination during the initial operating phase of the engine and wherein the modulation is a function of such determination during the subsequent operating phase of the engine.
In one embodiment, the engine including a sensor system for producing a signal indicative of an air/fuel ratio operating condition of the engine wherein, during an initial, relatively low temperature operating phase of the engine, the engine operates with a modulated air/fuel ratio established independently of the signal and wherein, during a subsequent higher temperature operating phase of the engine, the air/fuel ratio is modulated in as a function of the signal.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.